For an average character project, the tiles *could* be broken up into the head in one tile, the front of the body on another and the back on another still. Decisions are made at this stage as to how the model will be divided, typically based on what degree of resolution is required for the project. Using a UDIM workflow begins with the UV mapping of the actual geometry. This illustration demonstrates a small section of the overall UDIM tile area as it relates to the UV space (the upper image corresponding to the lower left corner of this one). The numbering starts at the UV origin and moves in the positive direction outward ten tiles (to 1010) and then repeats the rows stacked on top of each other moving upward 9999 rows.
The UDIM workflow is simply a notation format that names images accordingly so they are automatically assigned to a specific UV tile. Each image can have its own resolution as well. With a UDIM workflow, each whole tile section, 0-1, 1-2, 2-3 and so on, can have its own independent image map assigned to it, all within the same surface. Because of this fact, traditionally only the UV 0-1 space was ever really used (well, outside of controlling how a surface tiled an image). The UV map coordinate values are organized into a grid system where the image itself appears in the 0-1 area of the grid and the area outside of this tile simply repeats the image outward. Each vertex of the originating surface is given a coordinate on the UV map and the pixel values in-between are interpolated across the flat polygon surface for highly accurate texturing control (with the 'U' and 'V' axes names chosen specifically as to not cause confusion with the X,Y and Z axes for the actual model). UV mapping is the translation of a three-dimensional surface into the two-dimensional coordinate system of flat bitmap images (or vice-versa depending on how you look at it).
In order to understand the benefits of the UDIM workflow, its best to have a basic understanding of how UV mapping works. At its core, UDIM is simply an automatic UV offset system that assigns an image onto a specific UV tile, that allows artists to use multiple lower resolution texture maps for neighboring surfaces, producing a higher resolution result without having to resort to using a single, ultra high-resolution image. UDIM is a enhancement to the UV mapping and texturing workflow that makes UV map generation easier and assigning textures simpler.